414 MICROORGANISMS 



many materials from their attacks by merely drying them. 

 It is by drying out when ripe and remaining dry for long times 

 that seeds of plants are able to avoid being consumed by 

 bacteria during the resting period before germination. Many 

 kinds of foods, such as fruits, vegetables, and the seeds of 

 plants can be kept indefinitely by drying them out and keeping 

 them dry. Building materials which are of plant origin, 

 such as lumber may be preserved indefinitely against the 

 attacks of bacteria if they can be kept dry. We paint our 

 houses largely for the purpose of enabling the lumber to shed 

 water and to remain dry, and thus to avoid the attacks of 

 bacteria. 



474. Temperature Required. Bacteria vary widely in their 

 temperature relations. Some are able to grow at almost the 

 freezing temperature, while others thrive at temperatures as 

 high as from 160 to 190F. Three temperature limits may 

 be distinguished for each bacterium as follows: a MINIMUM, 

 or the lowest temperature at which growth is possible; an 

 OPTIMUM, or the temperature at which they grow best; and 

 a MAXIMUM, or the highest temperature at which growth is 

 possible. In some species the range between the minimum 

 and maximum is wide while in others, it is comparatively 

 narrow. The minimum temperature of some species is higher 

 than the maximum of others. Considering this wide range 

 of temperature relations, it is obvious that it is very difficult 

 entirely to prevent the action of bacteria through the agency 

 of temperature in substances which are injured by freezing. 

 At very low temperatures, just above the freezing point, very 

 few bacteria are active and none are very active. This fact 

 makes possible the preservation of many kinds of food for 

 comparatively long times by the method of cold storage. 



475. Necessary Chemical Conditions. Many chemical 

 substances have been found to be detrimental to, or destructive 

 of, bacterial life. Most bacteria, for example, do not thrive 

 in acid media and on this fact rests the preserving power of 

 vinegar and other organic acids. Common salt, saltpeter, 



